408 
CRUSTACEA. 
ORDER SIPHONOSTOMATA. 
Genus Argulus. 
A.foliaceus, Jurine. 
Belfast, Oct. 29, 1838. — In our market to-day I had the pleasure of 
detecting one of these very interesting and handsome parasites attached 
to the dorsal fin of a Salmo Trutta, about a foot in length. The Argulus 
is 3^ lines long, is a female, and in addition to the ova exhibits at the 
base of the tail the dark green spots (“ noirs,” Desm. Consid. Gen. Crust., 
p. 332) which are considered to mark this sex. Although the fish to 
which it was attached had been for some hours out of the water, the Argu- 
lus held so firmly by its two disks that. I had some difficulty in detaching 
it without injury. For about ten minutes it was wrapped in a piece of 
dry paper, and then placed in a vessel of water in which salt had been 
dissolved until it was to the taste like strong sea-water.* This was no 
sooner done than my pretty captive, after drawing her last pair of feet 
together several times, t thus calling to mind the common housefly, struck 
out her oarfe, and thereby was rapidly impelled through the fluid. 
The figures of Desmarest (tab. 50) and Yarrell (Brit. Fish., vol. ii. p. 
399) are very characteristic of this species, but the great beauty exhibited 
in the specimen before me is at the same time not shown, perhaps in con- 
sequence of the upper side of the female not being represented — this con- 
sists in its being closely spotted with very dark green along the central 
part of the body for two-thirds posteriorly, commencing a little above the 
ovary in the form of a head, and extending to the posterior portion ; the 
rest of the upper side of the body being of a very pale yellowish green hue 
and semi-transparent as described, the part thus spotted is well defined, 
and is strikingly of the form of a coleopterous insect, which the Argulus 
in another point of view resembles, when the two sides of the greenishly 
transparent “ boucliers ” are thrown a little apart, as we see the elytra of 
the insect. I was further reminded of the resemblance when attempting 
to remove it, as in holding firmly by the suckers, the body was drawn in, 
and the “ boucliers ” elevated quite above it. Its motion through the 
water seems equally rapid, whether it be on the upper or under side, or 
swimming retrally — it frequently moved along the surface with its back 
downwards, and was wholly immersed except the suckers, which were 
thrown either on a line with the water or quite above it, and thus would 
the animal occasionally remain quiescent for a short period. 
The constant motion of these organs (visible to the naked eye) in addi- 
tion to the rapid play of the feet, impart much life to the appearance of 
the Argulus, and present not the same aspect for two continuous seconds 
of time, whether the body be at rest or otherwise. They — i. e. the mar- 
ginal row of minute suckers, which appear as a dark line round the 
disk in figures of the species — are frequently drawn together to the 
centre of the disk, exhibiting a dark point not larger than the eye. 
* This was done in consequence of my having been told that the fish was 
taken in the sea ; the stomach, however, contained the remains of fresh-water 
insects (according to my friend A. H. Haliday, Esq., to whose inspection they 
were submitted) , which possibly might have been washed into the sea and there 
obtained, but this is by no means probable. 
f I observed this repeatedly done afterwards— they seem to be rubbed against 
the caudal plates. 
